Tube Mic Design - "Why is it done like this..."

[daschnoz]

Sorry if this has been asked. I did search, but didn't find anything relevant.


Spending 25 years in I.T. and software development, if the answer is "because that's the way we've always done it", I start looking for a better way. I may find that the current way is the best, but I at least look.

I'll lead with the question - Why are tube mic circuits AC coupled to the output transformer?

I spent a few years playing gigs with a tube guitar amp that I designed and built. The power side was single-ended. In that application, the x-fmr primary is the plate load. It does carry DC current, so the core needs to be designed to not saturate due to that current. In my design, I had around 175mA being pulled through that winding (parallel 6L6s). For an AT7 or AY7 single triode, we're looking at 1-2mA at best, sometimes far less. Using the power amp topology in a mic would eliminate the cap and its resonance thing with the x-fmr, phase shift, sonic coloring, etc. So I'm wondering why... Do the cores saturate that quickly in these small x-fmrs? Is this topology too noisy for a microphone application? I can't imagine it hasn't been tried before, so there has got to be a reason. ...or is this just the way it's always been done, so no one tried anything different?

 

[Khron]

Do the cores saturate that quickly in these small x-fmrs?

Very likely.

In that application, the x-fmr primary is the plate load. It does carry DC current, so the core needs to be designed to not saturate due to that current. In my design, I had around 15-20mA being pulled through that winding (parallel 6L6s).

Is that transformer gapped or not?

 

[daschnoz]

Very likely.



Is that transformer gapped or not?

Yes, a Hammond 125GSE, so it is designed for SE operation.
...and a correction made in the OP - more like 175mA (As I thought about it more, 15-20mA seemed too low. Stupid decimal point... That's what happens when I math in the morning.)

 

[Khron]

Yes, a Hammond 125GSE, so it is designed for SE operation.

Well, compare that 6lb "behemoth" to the little T14/1 used in C12 / ELA M251...

Bottom line being, there's no meaningful advantage to making transformers so tiny, and so complex (gapped core), for the arguable benefit of skipping one relatively bog-standard component. Especially since there's no notable amount of power / current needing transferring...

 

[RuudNL]

Why are tube mic circuits AC coupled to the output transformer?

Because we don't want DC through a (usually) non-gapped transformer.

 

[rock soderstrom]

Because we don't want DC through a (usually) non-gapped transformer.

+1

...and a gapped transformer with comparable (clean) low frequency response is much larger which can be a disadvantage in a tight microphone.

In principle, it would work...

 

[daschnoz]

So there is a reason... and it's one that makes sense.

Thanks guys.

Well, compare that 6lb "behemoth" to the little T14/1 used in C12 / ELA M251...

{snip}

So a 6 pound x-fmr in a tube mic, who cares. (that is quite a beastly hunk of iron)
The RCA 44 is a chunky 8 pounds, but everyone would love to have one of those in the mic locker.

 

[Rusan]

No doubt, an air-gapped Permalloy core with sufficient primary inductance at the lowest frequencies of interest would be a real hoss compared to an ungapped one, considering how easily it saturates. But, there's perhaps a more critical issue at hand. A larger lamination size/stack thickness carries with it the unwanted baggage of increased interwinding capacitance, leakage inductance, DCR, hysteresis loss, etc. Not exactly an ideal situation when pristine microdetail and low distortion are the goals.

Also, we're dealing with a tiny signal that's the genesis of the entire audio chain, and every anomaly and artifact will be magnified a thousand times down the line. Compare that to a single ended tube amplifier, with huge voltage/current swings that also happen to be located at the very tail end of the signal chain. Even at that, consider that parafeed single ended power amps with zero offset DC in the output iron had their heyday in the audiophile world, and were once quite popular for a good reason.

Of course, there's also the issue of noise, which you mentioned. Power supply rejection is virtually non-existent, especially with low plate resistance triodes. So, you must either inject some of the power supply noise in opposing phase or else brute-force it clean with multiple series pi filters.

Sorry to be pessimistic and naysay when I myself haven't actually tried it in a mic, but I have done it with single ended tube preamps, and all the above applies. I can only think it would be much worse considering that a microphone's output signal is an order of magnitude smaller, and will be amplified much more and then critically recorded, rather than just for music playback in a living room. I do understand it's not exactly comparing apples to apples, but it goes back to the original question of, "Why hasn't it been done?" Maybe it has been... An interesting concept regardless; would be fun to at least try!

 

[daschnoz]

{snip}

I do understand it's not exactly comparing apples to apples, but it goes back to the original question of, "Why hasn't it been done?" Maybe it has been... An interesting concept regardless; would be fun to at least try!

Yeah, maybe it has been done and failed miserably. The problem is that the people who tried it did so decades ago and have since left us, so we can't ask them about it. And since the idea didn't work, they likely didn't keep any notes on the project. They just knew it didn't work, so they never did it again. I'm sure we've all had a few projects like that.

 

[rock soderstrom]

But, there's perhaps a more critical issue at hand. A larger lamination size/stack thickness carries with it the unwanted baggage of increased interwinding capacitance, leakage inductance, DCR, hysteresis loss, etc. Not exactly an ideal situation when pristine microdetail and low distortion are the goals.

+1

I had already thought about that and I think that's at least an aggravating factor.

The required high step-down ratio of such transformers do not make things any easier. I think we have summarised the reasons why such arrangements have never (or very rarely?) been seen in the last 100 years.

I still think it could work if money, space and effort were not an issue...

 

[gyraf]

..we want as physically small transformer cores as possible in mics, as we are also fighting barkhausen quantitazion distortions at those very-very low levels involved. DC-coupled and thus gapped would probably be counterproductive at these levels..